High resolution electron beam systems include scanning electron microscopes, defect detection tools, VLSI testing tools and mask making and wafer exposure tools. In general, these systems include an electron source and electron optics for accelerating the electrons into an electron beam and focusing the electrons onto a target. These systems require an electron source with a high brightness and a small source size. Field emission electron sources have frequently been used in high resolution electron beam systems.
Negative electron affinity (NEA) photocathode electron sources have been proposed in the prior art. A negative electron affinity photocathode includes a semiconductor, usually a III-V compound such as gallium arsenide, heavily p-doped so as to raise the conduction band relative to the Fermi level. The semiconductor surface is coated with an activation layer a few monolayers thick, which lowers the work function so that the conduction band in the bulk is above the vacuum level, a condition of negative electron affinity. When electrons are excited by light energy into the conduction band within a diffusion length (typically a few micrometers) of the surface, many of them will diffuse to the surface where they will have a high probability of escaping into the vacuum, as described by R. L. Bell in Negative Electron Affinity Devices, Clarendon Press, New York, 1973.
In operation, a small diameter light beam illuminates an active area of the cathode, causing electrons to be emitted. Typically, a high brightness electron source is achieved by providing a small active emission area on the cathode. The active emission area is usually small in comparison with the total area of the emission surface of the cathode. Cathodes of this type, wherein electrons are emitted from a relatively small active area of a relatively large area cathode, are referred to herein as broad area cathodes.
In many cases, a broad area cathode is highly sensitive to contamination and may provide only limited lifetime when exposed to the many sources of contamination in an electron beam source. The replacement of a cathode in an electron beam system is frequently a lengthy, risky process, so that longer periods of operation with a cathode represent a significant technical advantage.
In a high vacuum or ultra high vacuum system, contamination of a cathode can arise from several sources, including: (1) normal outgassing, consisting of desorption of gases from surfaces within the system and diffusion of gases from the interior of objects in the system; (2) electron-stimulated desorption in which electrons striking a surface causes gases to be released, both as ions and as neutral particles; and (3) backstreaming of gases from parts of the system at poorer vacuum levels. In an electron beam column, all three of these sources create gases that may contaminate the cathode. The emission surface of the cathode cannot be completely physically shielded from the sources of contamination, because the electrons must pass from the emitting area to the column. It is possible to create electric fields that block most ions from reaching the cathode. It is also possible to use a sector magnet in the electron beam column to eliminate line-of-sight between the cathode and most sources of neutral contaminants. However, both approaches, particularly the sector magnet, add considerable complexity and expense to the system.
A further problem with NEA photocathodes relates to the cesium that is typically used to activate the photocathode surface. Cesium that migrates from the cathode to other parts of the electron beam column facilitates electrical breakdown of insulators and lowers the work functions of almost all materials, thereby increasing the risk of vacuum electrical breakdown between electrodes at different voltages.
Prior art electron beam columns have utilized an anode with a fairly large opening directly over the cathode. However, because of the voltage applied and the limitations on the maximum electric field that can be applied, the anode-cathode distance is such that very little shielding from the cathode from the column is effected and, in any case, the cathode is not protected from arcs, ions, gases released by ions, dark current and electrons emitted due to stray light. Furthermore, any cesium or other activating material is not contained in the region around the cathode surface, and insulators are not protected.
A scanned electron beam system, including an electron beam source using an NEA activated photoemitter as the cathode, is disclosed in U.S. Pat. No. 4,820,927 issued Apr. 11, 1989 to Langner et al. In the disclosed electron source, the anode is located directed above the cathode, so that a strong positive field is applied to non-emitting areas of the cathode. The non-emitting areas are exposed to virtually the same contamination as the emitting area. Electron beam sources utilizing photocathodes are also disclosed in U.S. Pat. No. 4,460,831 issued Jul. 17, 1984 to Oettinger et al; U.S. Pat. No. 4,970,392 issued Nov. 13, 1990 to Oettinger et al; U.S. Pat. No. 5,039,862 issued Aug. 13, 1991 to Smith et al; and U.S. Pat. No. 4,906,894 issued Mar. 6, 1990 to Miyawaki et al.
In the disclosed electron sources, the emitting and non-emitting areas of the cathode are exposed to contamination, and components of the electron beam source, such as insulators and electrode surfaces, may be contaminated by cathode materials. It is desirable to provide electron beam sources wherein these problems and disadvantages are overcome.